1. Field of the Invention
The present invention relates to a material for a fuel electrode of a solid oxide fuel cell, a fuel electrode for a solid oxide fuel cell, and a solid oxide fuel cell.
2. Description of Related Art
A solid oxide fuel cell (hereinafter referred to as “SOFC”) is a fuel cell in which a solid electrolyte exhibiting oxide ion conductivity is used as an electrolyte. The basic elements of the SOFC are a fuel electrode, a solid electrolyte and an air electrode, and an assembly which is prepared by laminating these three elements in order and bonding them to form a single cell. In general, a plurality of such single cells are used to form a stack, which is employed as a power generation device.
In the SOFC having such constitution, when a fuel gas (such as hydrogen, carbon monoxide and methane) and an oxidant gas (such as air and oxygen) are supplied to the fuel electrode and the air electrode, respectively, a difference arises between oxygen partial pressure on the part of the air electrode and that on the part of the fuel electrode; therefore, oxygen changes into oxide ions in the air electrode to be moved to the part of the fuel electrode through the solid electrolyte, and the oxide ions reaching the fuel electrode react with the fuel gas to release electrons. Accordingly, connecting a load to the fuel electrode and the air electrode enables the Gibbs free energy change in a cell reaction to be directly produced as electrical energy to perform power generation.
Among the elements constituting the respective parts of the SOFC, the fuel electrode is a field of electrochemical oxidation of the fuel gas as well as a field of generation of electrons. Since there is a necessity to operate for a long time under high temperatures (700-1000° C.) and low oxygen partial pressure, the fuel electrode is generally required to have the following properties.
Specifically, the properties include (1) chemical and thermodynamical stability under reducing atmosphere; (2) high catalytic activity; (3) high electric conductivity; (4) high sintering resistance and maintainability of a porous structure; and (5) an equal or approximately equal thermal expansion coefficient with respect to the solid electrolyte.
Conventionally, as a fuel electrode material meeting these properties, nickel powder or nickel oxide powder (nickel oxide changes into nickel when exposed under reducing atmosphere at high temperatures) is frequently used; however, using only nickel powder or nickel oxide powder causes such a problem that nickel particles are mutually sintered when used at high temperatures for a long time, making it hard to maintain a porous structure.
Hence, for example, in order to suppress the mutual sintering of the nickel particles, a mixed powder of nickel powder or nickel oxide powder and a solid electrolyte powder such as yttria-stabilized zirconia (YSZ) (a sintered body of the mixed powder is called a cermet) has frequently been used in recent years.
Additionally, for example, Japanese Patent Application Unexamined Publication No. Hei9-245817 discloses that a surface reforming powder of particles, where a surface part is metallic nickel and titanium oxide is present inside the surface part as an irregularly shaped core, is used as the fuel electrode material in order to prevent degradation in generating performance of the SOFC that is caused by degradation of the fuel electrode due to the sintering of nickel.
In general, the fuel electrode of the SOFC is used under a reducing atmosphere by the fuel gas supplied in the state of power generation. Therefore, nickel in the fuel electrode exists as metallic nickel, which forms, in this state, a network structure where nickel particles are mutually connected, and the network structure acts as an electrically conducting path for the electrons generated in the fuel electrode.
However, when air flows into the fuel electrode due to an abrupt shutdown of the SOFC, a breakdown of a fuel line, or the like, the nickel is oxidized to change into nickel oxide, causing a volumetric expansion. Therefore, the fuel electrode prepared by sintering conventionally known fuel electrode materials has a problem in that the network structure which has been constructed under a reducing atmosphere breaks down when the fuel gas is supplied again to reduce nickel oxide back to nickel, causing performance degradation of the fuel electrode.
Additionally, at startup and shutdown of the SOFC, it generally becomes necessary to perform operations such as fuel purging using an inert gas such as a nitrogen gas in order to maintain a reduction state of the fuel electrode. For this purpose, the SOFC should be regularly equipped with a high-pressure nitrogen cylinder and the like.
However, the regular equipment of the SOFC with the high-pressure nitrogen cylinder and the like raises security concerns and problems such as complicated maintenance of the SOFC. In particular, assuming that the SOFC prevails as a distributed power source intended for home use in the future, it is not practical for every home to be equipped with a nitrogen cylinder.